1. Technical Field of the Invention
The present invention relates to an engine.
2. Description of Related Art
An example of the conventional engines, as shown in FIG. 11, arranges a pair of upper and lower rotary balancers 104a and 104b on a right horizontal side of a crank shaft 103 and provides an opening 141 of a cylinder block 122 on a right horizontal side of the paired upper and lower rotary balancers 104a and 104b, which opening is closed by a closure 142, at a specific observation position with a cylinder 102 oriented horizontal on a left side of a center axis 101 of the crank shaft 103, when seen in a direction parallel to the center axis 101 of the crank shaft 103 as well as the present invention.
As shown in FIG. 11, at the specific observation position, with a piston 124 existing at a bottom dead center, the pair of the upper and lower rotary balancers 104a and 104b have weight connection portions 115a and 115b positioned on respective left horizontal sides of their rotation centers 143a and 143b. A gap 144 is formed between the both weight connection portions 115a and 115b when the piston 124 exists at the bottom dead center.
As shown in FIG. 14, at the specific observation position, with a larger-diameter end portion 107 of a connecting rod being present at a stroke intermediate position above the center axis 101 of the crank shaft 103, while the upper rotary balancer 104a has the weight connection portion 115a positioned above the center 143a of its rotation, the lower rotary balancer 104b has the weight connection portion 115b disposed below the center 143b of its rotation. A gap 146 is formed between the both weight connection portions 115a and 115b when the larger-diameter end portion 107 is present at the stroke intermediate position.
This conventional technique is different from the present invention on the following points.
As shown in FIG. 11, at the specific observation position, with a center axis 106 of the connecting rod oriented horizontal, a pair of upper and lower cap segment surfaces 109a and 109b of the larger-diameter end portion 107 of the connecting rod are positioned at an upper and a lower portions of a crank pin 108 and are made vertical. A cap 110 is attached by a pair of an upper and a lower connecting rod bolts 111a and 111b, head portions 112a and 112b of which are arranged vertically along the paired upper and lower cap segment surfaces 109a and 109b. 
And as shown in FIG. 12, at the specific observation position, with the piston 124 existing at a position just after it has passed the bottom dead center, while the upper rotary balancer 104a has the weight connection portion 115a arranged at a left lower portion of the rotation center 143a, the lower rotary balancer 104b has the weight connection portion 115b disposed at a left upper portion of the rotation center 143b. A gap 144a is formed between the both weight connection portions 115a and 115b just after the piston 124 has passed the bottom dead center. The upper connecting rod bolt 111a has the head portion 112a opposed to the gap 144a from its right lower portion and has an axis an extension line 145a of which goes rightwards and downwards to pass through the gap 144a and the opening 141.
As shown in FIG. 13, at the specific observation position, with the piston 124 being present at a position just before it passes the bottom dead center, while the upper rotary balancer 104a has the weight connection portion 115a arranged at a left upper portion of its rotation center 143a, the lower rotary balancer 104b has the weight connection portion 115b positioned at a left lower portion of its rotation center 143b. A gap 144b is formed between the both weight connection portions 115a and 115b just before the piston 124 passes the bottom dead center. The lower connecting rod bolt 111b has the head portion 112b opposed to the gap 144b from its right upper portion. This lower connecting rod bolt 111b has an axis an extension line 145b of which goes rightwards and upwards to pass through the gap 144b and the opening 141.
And as shown in FIG. 14, at the specific observation position, with the larger-diameter end portion 107 of the connecting rod existing at an stroke intermediate position above the center axis 101 of the crank shaft 103, the lower connecting rod bolt 111b has the head portion 112b opposed to the gap 146 from its right portion. This connecting rod bolt 111b has the axis the extension line 145b of which goes rightwards and upwards to interfere with the upper weight connection portion 115a. 
This conventional technique has the following problems.
<Problem 1> There Occurs Stagnation in the Work for Attaching and Detaching the Cap.
As shown in FIGS. 12 to 14, when conducting a predetermined maintenance work of the piston 124 or the like, there is a case where the closure 142 is removed and the paired connecting rod bolts 111a and 111b are removed by a socket wrench 147 to detach the cap 110 while retaining the paired rotary balancers 104a and 104b assembled to the cylinder block 122, and after the predetermined maintenance work has been completed, the cap 110 is attached to assemble the connecting rod bolts 111a and 111b by the socket wrench 147. In such a case, the lower connecting rod bolt 111b cannot be detached and attached at once to result in stagnating the work for attaching and detaching the cap 110.
More specifically, in the case of detaching the cap 110, first, after the engine has been brought into such a state that the piston 124 has just passed the bottom dead center as shown in FIG. 12, the straight socket wrench 147 is inserted into the cylinder block 122 through the opening 141 and a socket 148 at its leading end is inserted from the gap 144a along the extension line 145a of the axis of the upper connecting rod bolt 111a. The upper connecting rod bolt 111a has the head portion 112a fitted into the socket 148. Then the socket wrench 147 is rotated to dismantle the upper connecting rod bolt 111a. Next, after the engine has been brought into such a state that the piston 124 is just before passing the bottom dead center as shown in FIG. 13, the same socket wrench 147 is inserted through the opening 141 and the socket 148 at its leading end is inserted from the gap 144b along the extension line 145b of the axis of the lower connecting rod bolt 111b and the head portion 112b of the lower connecting rod bolt 111b is fitted into the socket 148. The socket wrench 147 is rotated to once loosen the lower connecting rod bolt 111b. Subsequently, after the engine has been brought into the stroke intermediate state as shown in FIG. 14, both hands are inserted from the opening 141 to the cylinder block 122 via the gap 146. While supporting the cap 110 by one hand, the lower connecting rod bolt 111b is dismantled by the other hand.
Next, in the case where the cap 110 is attached after having conducted the predetermined maintenance work, first, the engine is brought into the stroke intermediate state as shown in FIG. 14. Then the cap 110 and the lower connecting rod bolt 111b are held by both hands and are inserted from the opening 141 to the crank pin 108 via the gap 146. While supporting the cap 110 by one hand so that it dose not fall, the lower connecting rod bolt 111b is once temporarily fastened by the other hand. Subsequently, after having brought the engine into such a state that the piston 124 is just before passing the bottom dead center as shown in FIG. 13, the straight socket wrench 147 is inserted from the opening 141 into the cylinder block 122 and the socket 148 at the leading end is inserted from the gap 144b along the extension line 145b of the axis of the lower connecting rod bolt 111b. The head portion 112b of the lower connecting rod bolt 111b is fitted into the socket 148 and the socket wrench 147 is rotated to rigidly fasten the lower connecting rod bolt 111b. Next, the engine is brought into such a state that the piston 124 has just passed the bottom dead center as shown in FIG. 12 and thereafter the same socket wrench 147 has the socket 148 at the leading end, fitted onto the head portion 112a of the upper connecting rod bolt 111a, which is inserted into the cylinder block 122 through the opening 141. The socket 148 at the leading end is inserted from the gap 144a to the cap 110 with the upper connecting rod bolt 111a. The upper connecting rod bolt 111a is attached.
<Problem 2> The Engine is Produced at a High Cost.
When an engine of high exhaust amount is produced by borrowing a cylinder block 122 to be used for an engine of low exhaust amount and making a radius 123 from the center axis 101 of the crank shaft to a center of the crank pin 108 larger than that of the engine of low exhaust amount and making a dimension 127 from a center of a piston pin 125 to a piston head top surface 126 smaller than that of the engine of low exhaust amount, if the engine of high exhaust amount maintains such a structure that the cap segment surfaces 109a and 109b form a vertical surface as well as the engine of low exhaust amount, while the engine is in operation, the head portion 112a of the upper connecting rod bolt 111a interferes with an upper portion 128a of the crank shaft 103 in the crank case 128. Therefore, conventionally, in the case of producing the engine of high exhaust amount, there was prepared a separate cylinder block which is exclusively used for the engine of high exhaust amount and prevents the head portion 112a of the upper connecting rod bolt 111a from interfering with the upper portion 128a, without using the cylinder block 122 to be used for the engine of low exhaust amount. This entails a high production cost of the engine of high exhaust amount.
The present invention has an object to provide an engine capable of solving the problems. More specifically, the present invention provides an engine which enables a cap to be attached and detached quickly and can reduce the production cost.